1. Field of the Invention
The present application relates generally to the field of logging tools, and particularly to using logging tools to make measurements in virgin formations.
2. Background
Logging tools have long been used to make subsurface measurements. Most of the measurements made by commercial borehole logging tools have depths of investigation (DOI) of a few inches or less. A widely used metric for DOI for many borehole logging devices is the radial distance from the borehole wall from which 50% of the measured signal is received. The DOI is defined differently, however, for nuclear magnetic resonance (NMR) tools such as Schlumberger's MR Scanner™ tool. For those tool-types, all of the signal is derived from thin radial shells that are located different distances (e.g., 1, 2, 3, 4 in.) from the borehole wall.
Wellbores are usually drilled in an over-balanced condition. That is, the hydraulic pressure in the wellbore is maintained above the formation pore pressure. The near wellbore region is often contaminated by drilling fluid and damaged by fines invasion that occurs during the drilling process. Also, the combined action of drilling and particle filtration in the near wellbore region may result in a damaged zone having a permeability that is not representative of the rest of the formation. The region of the formation invaded by drilling fluids is called the invaded or flushed zone. There is also a particle filtrate layer, called “mudcake”, that forms during the drilling fluid invasion process that can adversely affect logging tool responses of pad-type devices.
Borehole logging measurements include those made by NMR, borehole imaging, resistivity, microwave dielectric, acoustic, nuclear, pressure, and fluid sampling tools. With the exception of electrical resistivity (e.g., deep laterolog tools) and conductivity (e.g., deep induction tools), borehole logging measurements are confined to the flushed zone; that is, within a few inches of the borehole wall. Fundamental physics and instrument limitations preclude most borehole logging measurements from penetrating deep enough into the formation to extend beyond the flushed zone. Measurements having a shallow DOI (e.g., less than 4 inches), such as NMR measurements, measure formation properties of the invaded/damaged zone that are not necessarily representative of the formation beyond the invaded/damaged zone.
The use of Pulsed NMR well logging measurements to evaluate oil and gas reservoirs began in the early 1990s. Since that time the technologies and interpretation methods have evolved to a high level of sophistication and today's tools can provide a host of reservoir fluid and rock properties. These include lithology independent total porosity, free and bound fluid water volumes, fluid types and saturations, and estimates of oil viscosity and reservoir permeability. NMR measurements can be made at different depths of investigation by changing the tool frequency; however, the DOI are generally limited to no more than a few inches beyond the borehole wall. The technology required to make NMR measurements at different depths is very complex.
As stated, Pulsed NMR well logging measurements have shallow depths of investigation and therefore only measure the flushed zone of the reservoir within a few inches of the borehole wall. Deep borehole NMR measurements do not seem possible because of the short range nature of static magnetic fields and the low signal to noise ratio of deep reading NMR measurements. It would be a significant advance in formation evaluation if shallow reading borehole measurements could be made in a non-invaded formation, i.e., beyond the depth of drilling fluid filtrate and fines invasion. The invasion or flushed zone radius depends on many factors including fluid loss and rheological properties of the drilling fluid, drilling rate, formation permeability and porosity, and the difference between hydrostatic drilling fluid pressure and formation pressure. Depending on those factors, flushed zone radii can vary from essentially nil to 10 feet in extreme cases. In wells drilled with synthetic oil base mud (SOBM), the flushed zone can be relatively shallow (e.g., 4 inches or less). The present disclosure provides a method and apparatus for performing measurements in the non-invaded or unaltered part of a formation.